The present invention relates generally to the field of olefin polymerization catalysis, catalyst compositions, methods for the polymerization and copolymerization of olefins, and polyolefins.
Polyolefin homopolymers, copolymers, terpolymers, etc., can be produced using various combinations of catalyst systems and polymerization processes. One method that can be used to produce such polyolefins employs a metallocene-based catalyst system. Generally, metallocene catalysts produce polyolefins with a narrow molecular weight distribution. While a polymer having a narrow molecular weight distribution can be advantageous in certain polymer processing operations and end-use applications, it can be a drawback in others. For instance, narrow molecular weight distribution polymers may require the use of a fluoropolymer additive in order to process the polymer at desirable production rates without flow instabilities, such as melt fracture, in some polymer processing operations. The use of a fluoropolymer processing aid increases the cost of producing a finished article from the polymer. Stability in other polymer processing operations, such as blown film and blow molding, often is reduced with a narrow molecular weight distribution polymer, as compared to broader molecular weight distribution polymers, resulting in reduced output or production rates.
Metallocene-based catalyst systems generally produce polymers with a narrow molecular weight distribution. Other catalyst systems, such as chromium or Ziegler-type, can produce broader molecular weight distribution polymers. However, when either a chromium or a Ziegler-type catalyst system is employed, the use of hydrogen in olefin polymerizations causes a narrowing of the molecular weight distribution of the polymer.
Hence, it would be beneficial to produce, in the presence of hydrogen, a broader molecular weight distribution polymer using a metallocene-based catalyst system. Accordingly, it is to these ends that the present invention is directed.